AC Freezing Up in Portola Valley

AC Freezing Up in Portola Valley

An AC coil freezes when its surface stays below 32 degrees long enough for condensation on the fins to turn to frost instead of running off. The frost keeps insulating the coil, so it builds until it is a solid block of ice, and at that point the system stops cooling and starts melting all over whatever is underneath the air handler. In a Portola Valley home built into wooded acreage, that is often a closet unit above a finished room, so the ice becomes a ceiling problem fast.

Portola Valley's oak and redwood hills keep the climate mild through summer, so cooling load here is moderate and a freeze-up is rarely about the system working too hard. It is almost always airflow that fell off somewhere. Larger custom homes on wooded acreage tend to run multiple zones behind dampers, and the Eichler-influenced homes near Ladera often add ductless heads or through-slab runs to a house that was never built with a full duct system, which gives airflow more places to go wrong. A damper stuck closed, a return blocked in a rarely used wing, or a mini-split head with a clogged internal filter can all starve one coil while the rest of the house cools fine.

In my experience on these properties the fix is one part or one restriction on one branch, and the equipment itself is usually sound. We shut the affected unit down, let it thaw completely, then read the actual airflow and refrigerant numbers on that specific zone instead of guessing from a wet coil.

Common causes

Dirty or clogged air filter. The single most common cause. A loaded filter starves airflow to the coil until it drops below freezing. On a wooded Portola Valley property this can be a central filter or, on an Eichler-influenced retrofit, the mesh filter inside a ductless head that nobody thinks to clean. We replace or clean it, check the coil, and read static pressure to confirm airflow actually came back.

Closed or stuck zone damper. Larger custom homes on acreage here often run zoned systems, and a damper that fails closed cuts airflow to its coil while the rest of the house keeps running. We test each damper motor and the zone board, confirm they open on call, and verify minimum airflow so no zone gets shut off completely.

Low refrigerant from a leak. Low charge drops coil pressure and temperature until ice forms, often starting at the suction line. Adding refrigerant without finding the leak only postpones the next freeze-up. We leak-test, repair it, and set the charge to the manufacturer's target by subcooling or superheat.

Dirty evaporator coil. A coil coated in dust and biofilm insulates the fins and restricts airflow, which drops its temperature into freezing range. We see this on older systems in the wooded-acreage homes that have run for years without the coil being pulled and inspected. We clean it and recheck the temperature split.

Weak blower or a clogged mini-split filter. A blower motor losing speed, a bad capacitor, or on an Eichler-influenced retrofit a ductless head with a packed internal filter, all move less air than the coil needs and it ices. We read amp draw against spec, test the capacitor, and check any ductless filters before condemning a motor.

Stuck blower relay or control fault. If the compressor runs but the indoor blower or the ductless head's fan does not start, the coil freezes within minutes. A stuck relay, a bad blower control, or a thermostat wiring fault can do it. We confirm the fan actually energizes on a cooling call and trace the circuit instead of swapping parts on a guess.

How we diagnose it

• Shut the system off and let the coil thaw fully before taking any reading, so the numbers reflect real operation, not a block of ice.
• Check and replace the filter on the affected air handler or ductless head, then read static pressure to confirm airflow recovered.
• For zoned systems, we test each damper along with the zone board, since a stuck-closed damper can starve a single coil of airflow.
• Suction pressure, superheat, and subcooling readings tell us whether it's an airflow issue or a low refrigerant charge.
• Inspect the evaporator coil and blower or ductless fan, and confirm it energizes on a cooling call, before condensate finds a ceiling.

$99 diagnostic, credited toward the repair. You get a written quote before any work begins.

AC Freezing Up in Portola Valley: common questions

Nearby and related

AC Freezing Up near Portola Valley: Woodside · Menlo Park · Los Altos Hills .

This is usually a ac repair in Portola Valley job. See our ac repair overview or the Portola Valley service area.